In a spinning reel, the spool does not rotate when the fisherman is casting and retrieving. The momentum of the lure being cast simply pulls the line and uncoils it from the reel spool. Even when retrieving the line, the spool itself normally does not rotate because a drag mechanism is set to hold the spool against rotation. However, the drag mechanism can be overcome to allow the spool to rotate when a predetermined tension on the line is exceeded, thus enabling a hooked fish to take out line without breaking it off.
In a spinning reel, it is desirable to be able to preset the drag to accommodate a given tension in the line, but to have available secondary means for quickly adjusting the drag either up or down from the preset level, without changing the given setting. It is further desirable that such secondary means for adjusting drag be itself variable so that its range of adjustment can be varied to accommodate different fishing conditions.